The invention relates to a planar electrochemical probe to determine gas components in gas mixtures, and in particular to such a probe comprising a probe body constructed of laminated layers of solid electrolyte layers, with at least one gas chamber integrated into the probe body and a diffusion hole guided to the gas chamber.
Such a planar electrochemical probe which operates according to the polarographic measuring principle is disclosed, for example, in German Patent No. 38 11 713, wherein both anode and cathode are exposed to the gas mixture to be measured. The cathode is constituted as an inner pump electrode in a gas chamber that is integrated into the probe body. The gas chamber is coupled to the gas mixture via a diffusion hole. In the method disclosed in German patent 38 11 713, the probe element is made by printing on the solid electrolyte layers, which may comprise ceramic films, by way of silk-screen printing technology, by laminating the layers together and by subsequently sintering the layers. The diffusion hole leading to the gas chamber is punched into the solid electrolyte layers before laminating and sintering. Punched solid electrolyte layers result in a poor laminar composite in the vicinity of the diffusion hole. Since the diffusion hole is located in the region of active, functional layers, a poor laminar composite also implies that the functional reliability of the probe is affected.
Additionally, practice has shown that particles (incineration ashes of oil) from the exhaust gas of internal combustion engines are deposited at the functional layers, which also impairs functional reliability. Furthermore, silk-screen pastes are sometimes printed into the diffusion hole during silk-screen printing, which, in extreme cases, may lead to a solid plug, which, in turn, would render the probe inoperative.
It has already been proposed to extend the diffusion hole through the entire body of the probe. Such a configuration, however, considerably limits the stackability of functional layers. Additionally, this creates problems especially in broadband probes that operate with an air reference. In this latter configuration, the reference air duct in the region of the diffusion hole must be moved further back which means that the electrodes are no longer opposite each other, thereby increasing the probe's internal resistance. In addition, the heater cannot be configured to cover the entire surface and must also be sealed vis-a-vis the exhaust gas. This makes manufacture costly and also results in poor temperature distribution in the probe body.